In recent years, in a radio receiver such as a portable telephone and a One-Segment receiver, a wider bandwidth is being required. To achieve a receiving system of a wider bandwidth, a cancellation function of second-order inter-modulation distortion needs to be enhanced.
As a method that is very familiar in a cancellation system of second-order inter-modulation distortion, there is a method that is disclosed in non-patent literature 1. FIG. 1 is a block diagram showing a configuration of receiver 10 that is disclosed in non-patent literature 1. According to the method disclosed in non-patent literature 1, as shown in FIG. 1, a weighted reference signal is subtracted from an IF signal, thereby adjusting second-order inter-modulation distortion that is generated in mixer 11. A correction coefficient to be used for this weighting is determined by the following method.
Mixer 11 outputs a differential output signal to LPF (Low Pass Filter) 12, and outputs a common mode output signal to common mode detecting section 14. First, common mode detecting section 14 detects a common mode output signal from mixer 11, and LPF 15 performs a low pass filter process to this signal, and removes higher harmonic components of this signal. ADC (Analog to Digital Converter) 16 converts an output of LPF 15 to a digital signal, generates a reference signal of a digital region, and outputs the reference signal of the digital region to LMS 17.
Next, LPF 12 removes a higher harmonic signal from the differential output signal of mixer 11. ADC 13 converts an output of LPF 12 to a digital signal, generates an IF signal of the digital region, and outputs the IF signal of the digital region to LMS 17.
LMS (Least Mean Square) 17 obtains a correlation value between the reference signal of the digital region and the IF signal described above, and determines an optimal correction coefficient to be used to correct second-order inter-modulation distortion, by using an LMS algorithm.
As another cancellation system of second-order inter-modulation distortion, there is a method disclosed in Patent Literature 1. According to this system, as shown in FIG. 2, DC (direct current) detecting section 22 detects DC (direct current) components that are contained in the output of mixer 21. Receiver 20 injects a constant current corresponding to DC components from constant current source 23 into an output of mixer 21, thereby correcting the DC components of the mixer output and suppressing a second-order distortion.